PROJECT SUMMARY/ABSTRACT The clinical gold standard for the assessment of cardiac fibrosis is late gadolinium enhancement (LGE) cardiac MRI (CMR), which is acquired using an inversion-recovery T1-weighted sequence, 10-to-20 minutes after the administration of a gadolinium-based contrast agent (GBCA). Though LGE CMR is widely used in the clinical assessment of myocardial infarction, viability and cardiomyopathies, there are several concerns about GBCA. These include its association with nephrogenic systemic fibrosis (NSF) in patients with renal impairment; allergic reactions to GBCA in some patients; and recently, concerns related to GBCA accumulation in the brain. Thus, strategies for assessing myocardial fibrosis without administering GBCA are highly desirable. Quantitative CMR techniques have received interest as alternatives for identifying myocardial fibrosis without GBCA. Native T1 mapping has received attention, with studies indicating its utility in various cardiomyopathies. Its main drawback has been its limited sensitivity, where the reported changes in T1 values from fibrosis has been in a small range. Magnetization transfer (MT) imaging is another technique that has been investigated through the MT ratio (MTR), which is a non-quantitative metric that depends on acquisition parameters. Nonetheless, it has shown promising correlation with LGE, even when implemented through a simple ratio of two images acquired with different balanced SSFP sequence parameters. T1? relaxation mapping, which is based on rotating frame relaxations, maps motional regimes in the micro-to-millisecond scale and has also shown promise, with histological confirmation in a swine model, as well as correlation with LGE in vivo. However, its use in the heart has been limited to continuous-wave spin-lock preparation, making it susceptible to magnetic field inhomogeneities, and limiting the range of macromolecular motion that it can assess. In this proposal, we seek to develop novel quantitative CMR techniques to unleash the full potential of MT imaging and rotating frame relaxation in assessing myocardial fibrosis. For MT contrast, we will extend beyond MTR and quantify MT parameters, improving sensitivity and reproducibility. For rotating frame relaxations, we will develop improved quantification techniques using adiabatic T1p, and higher order relaxations based on Relaxation Along a Fictitious Field in the rotating frame of rank n (TRAFFn). These methods will allow probing of a wide range of macromolecular motion regimes. Neither of our ideas on MT or rotating frame relaxation imaging have been explored for imaging the human myocardium, but their potential has been highlighted in other anatomies. Successful completion of this project has the potential to transform the way CMR is performed for the assessment of myocardial fibrosis, eliminating the need for GBCA administration.